Turbocharger system with reduced thrust load

ABSTRACT

An exemplary compressor is provided. The compressor includes a plurality of blades, a hub defining a front surface and a back surface, and a first flow restriction structure provided at the back surface of the hub. The plurality of blades are arranged in a predefined manner on the front surface for receiving input air flow at a first pressure and compressing the input air flow to provide an output air flow at a second pressure higher than the first pressure. The first flow restriction member is configured for preventing at least a portion of the output air flow at the second pressure from entering into the back surface of the hub to reduce an air pressure at the back surface of the hub.

BACKGROUND

Embodiments of the disclosure relate generally to turbocharger system used for engines such as internal combustion engines and more particularly to an improved compressor of the turbocharger system and an improved multi-stage turbocharger system for thrust load reduction.

Turbocharger is a forced induction device used in an engine such as an internal combustion engine. In general, the turbocharger operates to allow more power to be produced from the internal combustion engine. The turbocharger typically includes a turbine and a compressor that are coupled to each other via a drive shaft. During operation, exhaust gas discharged from an exhaust manifold of the internal combustion engine drives the turbine to rotate which in turn drives the drive shaft and the compressor to rotate. The compressor then compresses input air flow at an atmospheric pressure and provides compressed air at a boosted pressure to the inlet of the internal combustion engine. Because the compressed air forced into the inlet of the internal combustion engine contains more oxygen content, the power produced by the internal combustion engine can be increased as more fuel can be combusted in the cylinders of the internal combustion engine.

The turbocharger also utilizes one or more bearing devices to support various loads applied to the drive shaft. For example, a thrust bearing is typically used to support a thrust load applied along an axial direction of the drive shaft. The thrust load can be generated either by a pressure distribution in the turbocharger or by the momentum of the flow in the turbocharger. Too large thrust load leads to a reduced life of the thrust bearing. Therefore, it is desirable to provide turbocharger systems capable of reducing the thrust load.

BRIEF DESCRIPTION

In accordance with one embodiment disclosed herein, a compressor is provided. The compressor includes a plurality of blades, a hub defining a front surface and a back surface, and a first flow restriction structure provided at the back surface of the hub. The plurality of blades are arranged in a predefined manner on the front surface for receiving input air flow at a first pressure and compressing the input air flow to provide an output air flow at a second pressure higher than the first pressure. The first flow restriction member is configured for preventing at least a portion of the output air flow at the second pressure from entering into the back surface of the hub to reduce an air pressure at the back surface of the hub.

In accordance with another embodiment disclosed herein, a turbocharger system for an internal combustion engine is provided. The turbocharger system includes a turbine, a compressor, and a thrust bearing. The turbine is in flow communication with an exhaust manifold of the internal combustion engine for receiving exhaust gas discharged from the exhaust manifold and is driven to rotate by the exhaust gas. The compressor is coupled to the turbine through a drive shaft. The compressor is driven to rotate by the drive shaft in response to a rotation of the turbine for supplying pressurized air to an intake of the internal combustion engine. The thrust bearing is attached to the drive shaft for supporting at least a thrust load applied along an axial direction of the drive shaft. The compressor includes a hub defining a back surface provided with a flow restriction member. The flow restriction member deflects a flow path of at least a portion of the pressurized air entering into the back surface at least once to create a pressure difference between two areas at least partially defined by the flow restriction member, and the pressure difference created by the flow restriction member causes the thrust load applied along the axial direction of the drive shaft to be reduced.

In accordance with another embodiment disclosed herein, a multi-stage turbocharger system for an internal combustion engine is provided. The multi-stage turbocharger includes a low-pressure stage and a high-pressure stage. The low-pressure stage includes a low-pressure turbine and a low-pressure compressor. The low-pressure compressor is capable of being driven by the low-pressure turbine to compress input air flow at a first air pressure and provide intermediate air flow at a second air pressure higher than the first air pressure. The high-pressure stage includes a high-pressure turbine and a high-pressure compressor. The high-pressure compressor is placed downstream of the low-pressure compressor. The high-pressure compressor is capable of being driven by the high-pressure turbine to compress at least a portion of the intermediate air flow provided from the low-pressure compressor and supply output air flow at a third air pressure higher than the second air pressure to an intake of the internal combustion engine. The high-pressure compressor is in flow communication with the low-pressure turbine.

DRAWINGS

These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a sectional view of a compressor in accordance with an exemplary embodiment of the present disclosure;

FIG. 2 is a perspective view of the compressor shown in FIG. 1 in accordance with an exemplary embodiment of the present disclosure;

FIG. 3 is a back side elevation view of the compressor shown in FIG. 1 in accordance with an exemplary embodiment of the present disclosure;

FIG. 4 is an enlarged view of a portion of the compressor shown in FIG. 1 operating in a first state in accordance with an exemplary embodiment of the present disclosure;

FIG. 5 is an enlarged view of a portion of the compressor shown in FIG. 1 operating in a second state in accordance with an exemplary embodiment of the present disclosure;

FIG. 6 illustrates a sectional view of a compressor in accordance with another exemplary embodiment of the present disclosure;

FIG. 7 illustrates a sectional view of a compressor in accordance with yet another exemplary embodiment of the present disclosure;

FIG. 8 illustrates a schematic block diagram of a single-stage turbocharger system used for an internal combustion engine in accordance with an exemplary embodiment of the present disclosure;

FIG. 9 illustrates a schematic block diagram of a two-stage turbocharger system used for an internal combustion engine in accordance with an exemplary embodiment of the present disclosure; and

FIG. 10 illustrates a schematic block diagram of a two-stage turbocharger system used for an internal combustion engine in accordance with another exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the one or more specific embodiments. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term “or” is meant to be inclusive and mean either any, several, or all of the listed items. The use of “including,” “comprising” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect.

Embodiments of the present disclosure generally relate to thrust load reduction for turbocharger systems. The turbocharger systems are used for improving efficiency of engines such as internal combustion engines. In one embodiment, a compressor with at least one flow restriction structure is provided. Specifically, the flow restriction structure is provided at the back surface for preventing at least a portion of pressurized air flow produced by the compressor from entering at the back surface. Thus, the air pressure at the back surface can be reduced. Reducing the back surface pressure results in a reduced thrust load applied at a thrust bearing. As a result, over-wearing problems of the thrust bearing can be avoided and the life of the thrust bearing can be extended. In another embodiment, a two-stage turbocharger system is provided. The two-stage turbocharger system includes a high-pressure stage turbocharger system and a low-pressure stage turbocharger system. In one implementation, at least a portion of the pressurized air flow produced by a high-pressure compressor of the high-pressure stage turbocharger system is diverted to a back surface of a low-pressure turbine of the low-pressure stage turbocharger system to increase the air pressure at the back surface of the low-pressure turbine. Thus, the net thrust load applied to a low-pressure thrust bearing in the low-pressure stage turbocharger system can be reduced to avoid over-wearing problems of the low-pressure thrust bearing and/or extend or prolong the life of the low-pressure thrust bearing.

Referring to FIG. 1, a sectional view of compressor 20 is illustrated in accordance with an exemplary embodiment of the present disclosure. The compressor 20 can be used in a turbocharger system for supplying pressurized air directly or indirectly to an engine. For example, the compressor 20 may be used in a single-stage turbocharger system 400 shown in FIG. 8 for supplying pressurized air directly to engine 440. For another example, the compressor 20 may be used in two-stage turbocharger systems 600, 700 shown in FIG. 9 or FIG. 10 respectively for supplying pressurized air to engine 602. It can be understood the compressor 20 can also be used in other multistage turbocharger systems. The single-stage turbocharger system 400 and the two-stage turbocharger systems 600, 700 will be described with more details below. Also, other than the turbocharger systems 600, 700 shown in FIGS. 8-10, one skilled in the art can contemplate that the compressor 20 with the one or more improved features described below, for example, features for thrust load reduction, can be equally used in other industrial applications, including but not limited to, gas turbines and steam turbines for example.

In one implementation, the compressor 20 shown in FIG. 1 may be a centrifugal compressor that can be driven to rotate at certain speed such that an input air flow 212 at a first air pressure received from a first air supplying source can be compressed to provide compressed/pressurized output air flow 214 at a second air pressure. The second air pressure is typically higher than the first air pressure. In one embodiment, the input air flow 212 can be received directly from the atmosphere and the first air pressure is the atmospheric pressure. In another embodiment, the input air flow 212 may be taken from an upstream compressor which supplies an output air flow at a boosted pressure higher than the atmospheric pressure.

Further referring to FIG. 1, in one implementation, the compressor 20 generally includes a hub 220 which is fixedly mounted to one end of a drive shaft 30. The other end of the drive shaft 30 may be coupled to a turbine (not shown in FIG. 1) which can be driven to rotate by exhaust gas discharged from an engine for example. The hub 220 can be driven to rotate around a rotational axis 302 in response to rotational movement of the turbine. In one implementation, as shown in FIG. 2 and FIG. 3, the hub 220 may define a center bore 229. One end of the drive shaft 30 can extend through the center bore 229 and is secured with the hub 220 via nuts or screws for example. The hub 220 generally defines a first surface 216 and a second surface 218. The first surface 216 is a front surface facing the input air flow 212. The first surface 216 is provided with a plurality of blades 215. The plurality of blades 215 can be spaced apart on the first surface 216 in a predetermined manner to define a plurality of air channels for the input air flow 212 to pass through. As shown in FIG. 1, the input air flow 212 may generally flow along a horizontal direction parallel to the axial direction 302 of the drive shaft 30. After compression, the output air flow 214 generally flows along the vertical direction or radial direction 210 of the compressor 20. The second surface 228 is a back surface or rear surface that is disposed adjacent to wall of a compressor housing 240. More specifically, a space is defined between the back surface 218 and the wall of the compressor housing 240 to allow the compressor 20 rotate without contacting the compressor housing 240.

Without a sealing structure arranged between the output air flow 214 and the space defined between the back surface 218 and wall of the compressor housing 240, a portion of the output air flow 214 or a leakage air flow 222 may enter into the space. The leakage air flow 222 in the space has an air pressure which generates a back-surface axial thrust force/load 217 pointing from the back surface 218 to the front surface 216. The back-surface axial thrust force/load 217 then is transmitted through the drive shaft 30 to a thrust bearing 304 attached to the drive shaft 30. Too large back-surface axial thrust force 217 may cause over-wearing problems of the thrust bearing 304 and may thus reduce the life of the thrust bearing 304. Therefore, to avoid over-wearing problems and/or to extend or prolong the life of the thrust bearing 304, it is desirable to reduce the amount of the leakage air flow 222 at the back surface 218 so as to reduce the axial thrust force/load 217 applied at the thrust bearing 304.

In one implementation, to reduce the amount of leakage airflow 222 at the back surface 218 or prevent at least a portion of the output air flow 214 from entering into the space at least partially enclosed by the back surface 218, a flow restriction structure 224 is introduced at the back surface 218. The flow restriction structure 224 generally divides the space into a first region 226 and a second region 228. The first region 226 is located adjacent to the edge of the compressor 20 where the output air flow 214 is produced. The second region 228 is located adjacent to a center of the compressor 20 where the drive shaft 30 is mounted. In general, the flow restriction structure 224 can be viewed as a flow deflection mechanism which functions to deflect or change a flow path of the leakage air flow 222 such that the leakage air flow 222 is made more difficult flowing from the first region 226 to the second region 228. The flow restriction structure 224 can also be viewed as a flow path extension mechanism which extends the flow path for the leakage air flow 222 to pass through. For example, the flow restriction structure 224 may create a non-linear flow path for the leakage air flow 222 to pass through. Due to flow deflection mechanism or the flow path extension mechanism, the amount of air flow in the second region 228 is less than that in the first region 226 or an air pressure difference is created between the first and second regions 226, 228. That is, the air pressure at the first region 226 is larger than that in the second region 228. As a result, a combined air pressure of the first region 226 and the second region 228 is reduced and the reduced air pressure leads to a reduced thrust load 217 applied at the thrust bearing 304.

Further referring to FIG. 1, in one implementation, the flow restriction structure 224 includes a first restriction section 223 and a second restriction section 225. In one implementation, the first restriction section 223 is a member that generally protrudes backwardly from the back surface 218 of the hub 220 and extends along the axial direction 302 of the drive shaft 30. The second restriction section 225 is a groove or recess defined in a wall of the compressor housing 240 for non-contactively receiving the first restriction section/member 223 therein. In other implementations, the first restriction section 223 and the second restriction section 225 may exchange roles. For example, the first restriction section 223 may be a member protruding forwardly from the wall of the compressor housing 240 and the second restriction section 224 is a groove or recess defined in the back surface 218 of the hub 220 for receiving the first restriction section 223 therein.

Further referring to FIG. 1, the first restriction section 223 may be formed integrally with the back surface 218 of the hub 220. In other implementations, the first restriction section 223 may be detachably coupled to the back surface 218 of the hub 220. Referring also to FIGS. 2 and 3, the first restriction section 223 may also extend along a circumferential direction 227 at the back surface 218 to form a ring-shaped member. In other implementations, the first restriction section 233 is not necessarily extending continuously along the circumferential direction 227 of the back surface 218. For example, the first restriction section 233 may include multiple elements separately arranged along the circumferential direction 227 of the back surface 218. In the illustrated embodiment, the ring-shaped first restriction section 223 divides the back surface 218 into a first region 226 and a second region 228.

Referring to FIG. 4, an enlarged view of a portion of the compressor 20 including the flow restriction structure 224 is shown in accordance with an exemplary embodiment of the present disclosure. More specifically, the first restriction section 223 of the flow restriction structure 224 defines a first surface 232, a second surface 234, and a third surface 236. The second restriction section 225 of the flow restriction structure 224 is a groove defined by a first wall 242, a second wall 244, and a third wall 246. As shown in FIG. 4, the leakage air flow 222 initially flows along a first channel 235 which is generally parallel to the radial direction 210 of the compressor 20. As used herein, “radial direction” is generally defined as a direction extending from a center at which a drive shaft 30 is mounted to an edge of the compressor 20 where the output air flow 214 is produced. The leakage air flow 222 is then deflected to flow in a second channel 237 defined between the first surface 232 and the first wall 242. The second channel 237 is substantially parallel to the axial direction 302 of the drive shaft 30. The leakage air flow 222 flowing in the second channel 237 is further deflected to flow in a third channel 239 defined between the second surface 234 and the second wall 244. The third channel 239 is substantially parallel to the radial direction 210. The leakage air flow 222 flowing in the third channel 239 is further deflected to flow in a fourth channel 241 defined between the third surface 236 and the third wall 246. The fourth channel 241 is substantially parallel to the axial direction 302 of the drive shaft 30. Due to the deflection mechanism, the leakage air flow 222 is made difficult to reach the second region 228 such that the air pressure at the back surface 218 of the compressor 20 can be reduced.

Further referring to FIG. 4 and FIG. 5, in which FIG. 4 shows the compressor 20 in a stationary state or a non-rotational state and FIG. 5 shows the compressor 20 in a rotational state. In this stationary state, the channel 237 defined between the first surface 232 and the first wall 242 has a first dimension of d₁. In the rotational state or when the compressor 20 is rotating, a centrifugal force applied to the first restriction section 223 causes the first restriction section 223 to move along the radial direction 210 and away from the center. That is, the first surface 232 tend to approach the first wall 242 of the compressor housing 240. As the compressor housing 240 remains stationary, the first channel 237 defined between the first surface 232 and the first wall 242 is reduced to have a second dimension of d₂ which is smaller than the first dimension d₁. The flow channel 237 with reduced dimension makes the leakage air flow even more difficult to reach the second region 228. As a result, the second region 228 can be substantially sealed with respect to the first region 226.

FIG. 6 illustrates a sectional view of a compressor 20 in accordance with another exemplary embodiment of the present disclosure. The embodiment shown in FIG. 6 is substantially similar to that has been described with reference to FIG. 1. Thus, similar elements will not be described with details in this embodiment. In the illustrated embodiment, the flow restriction member 224 is further configured for balancing purposes. During manufacturing process of the compressor, various factors such as irregularities in mass distribution can make the compressor unbalanced which means that rotational movement of the compressor 20 is substantially eccentric. Conventionally, scalloping means has been employed which removes material at the outer edge and between blades of the compressor wheel for balancing the compressor. However, scalloping the hub of the compressor can bring performance penalties to the compressor. For example, more output air flow may be leaked from the scalloped area to the back surface of the compressor. In the illustrated embodiment, balancing of the compressor 20 is achieved by removing material from the first restriction section 223 protruding backwardly at the back surface 218 of the compressor 20. The specific amount and location of the material to be removed from the first restriction section 223 is determined according to practical requirements. After balanced, the rotational movement of the compressor 20 can be substantially concentric. It should be understood that other than the weight-removal features as described herein, in some embodiments, the flow restriction member 224 may be added with some material for balancing the compressor 20. Also, the amount and location of the added material is determined according to the practical requirements.

As described with reference to FIGS. 1-3, the back surface 218 of the compressor 20 is provided with one flow restriction structure 224 used for reducing the air pressure at the back surface 218. However, to achieve air pressure reduction, the back surface 218 can be provided with more than one flow restriction structures. For example, FIG. 7 shows another embodiment of the compressor 20 in which two flow restriction structures are included. In the illustrated embodiment, the back surface 218 of the compressor 20 is provided with a first flow restriction structure 252 and a second flow restriction structure 254. The first and second flow restriction structures 252, 254 has configurations that are similar to the flow restriction structure 224 described above with reference to FIGS. 1-3. More specifically, the first flow restriction structure 252 and the second flow restriction structure 254 are spaced apart along the radial direction 210 and divide the back surface into a first region 226, a second region 227, and a third region 228. The first flow restriction member 252 deflects the leakage air flow 222 and creates an air pressure difference between the first region 226 and the second region 227. That is, the air pressure at the second region 227 is smaller than the first region 226. Further, the second flow restriction structure 254 deflects the leakage air flow 222 and creates an air pressure difference between the second region 227 and the third region 228. That is, the air pressure at the third region 228 is smaller than the second region 227. In addition, the radial movement of the first and second flow restriction structures 252, 254 with respect to the wall of the compressor housing 240 can further reduce the amount of leakage air flow 222 at the back surface 218 of the compressor. Thus, the air pressure at the back surface 218 can be significantly reduced thereby the axial thrust load 217 applied to the thrust bearing 304 can be reduced. Therefore, over-wearing problems of the thrust bearing 304 can be avoided and/or the life of the thrust bearing 304 can be prolonged or extended.

Further referring to FIG. 7, in some embodiments, either one or both of the first and second flow restriction members 252, 254 can be used for balancing of the compressor 20. More specifically, in one implementation, the first flow restriction structure 252 is partially removed with material for balancing. In another implementation, the second flow restriction structure 254 is partially removed with material for balancing. In yet another implementation, both the first and second flow restriction structures 252, 254 are removed with material for balancing. Still in some implementations, either one or both of the first and second flow restriction members 252, 254 can be added with material for balancing.

FIG. 8 illustrates a single-stage turbocharger system 400 in which the various compressor embodiments described above can be implemented. More specifically, the single-stage turbocharger system 400 includes a turbine 402 and a compressor 404 that are coupled to each other via a drive shaft 406. The compressor 404 can has substantially the same configuration as the compressor 20 described above with reference to FIGS. 1-7. For example, one or more flow restriction structures 224 can be provided at the back surface 452 of the compressor 404. The turbocharger system 400 further includes a thrust bearing 408 which is schematically shown as being attached to the drive shaft 406 for supporting the thrust load 456 applied to the drive shaft 406. The thrust bearing 408 is a known element in the art, and thus detailed description of the thrust bearing 408 is omitted here. In the illustrated embodiment, the thrust load 456 is a net thrust load pointing from the turbine 402 side to the compressor 404 side and is parallel to the axial direction 302 of the drive shaft 406. The net thrust load 456 includes at least a compressor back-surface thrust load 458 component which is generated due to the leakage air flow at the back surface 452 of the compressor 404. The compressor back-surface thrust load 458 points substantially at the same direction as that of the net thrust load 456. Thus, reducing the compressor back-surface thrust load 458 can lead to a reduction of the net thrust load 456.

Further referring to FIG. 8, the turbine 402 is placed downstream of the exhaust manifold 444 of the engine 440 (e.g., an internal combustion engine) for receiving exhaust gas discharged from the exhaust manifold 444 and routed through an exhaust channel 446. The exhaust gas passes through the turbine 402 and drives the turbine 402 to rotate. The turbine 402 then drives the shaft 406 and compressor 404 to rotate. In one embodiment, a portion of the exhaust gas passing through the turbine 402 is discharged directly to the environment. In another embodiment, the exhaust gas passing through the turbine 402 may be re-circulated.

Further referring to FIG. 8, the compressor 404 compresses input air flow 412 and produces output air flow 413 at boosted air pressure. In the illustrated embodiment, the output air flow 413 is routed to an intercooler 416 via a first channel 414. The intercooler 416 functions as a heat exchanger to remove heat from the output air flow 413 as a result of the compression process. The cooled output air flow is routed to an intake manifold 442 via a second channel 418. In other embodiments, the output air flow 413 produced from the compressor 404 may be directly routed to the intake manifold 442 of the engine 440 without intercooling.

During operation, the one or more flow restriction structures 454 provided at the back surface 452 of the compressor 404 functions to reduce the amount of leakage air flow entering at back surface 452 of the compressor 404. The reduced leakage air flow leads to a reduced compressor back-surface thrust load/force 458 and a reduced net thrust load 456 applied at the thrust bearing 408. As a result, over-wearing problems of the thrust bearing 408 can be avoided and/or the life of the thrust bearing 408 can be extended or prolonged.

Further referring to FIG. 8, in some embodiments, the one or more flow restriction structures 454 provided at the back surface 452 of the compressor 404 can be modified for balancing the compressor 20. For example, a portion of the flow restriction structure 454 can be removed for balancing. In another embodiment, the flow restriction structure 454 can be added with material for balancing.

In other implementations, the compressor 20 described with reference to FIGS. 1-7 can be used in a two-stage turbocharger system 600. The two-stage turbocharger system 600 is configured for supplying pressurized air to engine 602 to improve the efficiency of the engine 602. Referring to FIG. 9, in one embodiment, the engine 602 includes an internal combustion engine. In the illustrated embodiment, the internal combustion engine 602 includes a plurality of cylinders or combustion chambers 604 for combusting fuels and gas supplied through intake manifold 606. After combustion, the exhaust gas is discharged from the exhaust manifold 608.

Further referring to FIG. 9, the two-stage turbocharger system 600 includes a high-pressure stage turbocharger system 620 and a low-pressure stage turbocharger system 640 in flow communication with each other. The high-pressure stage turbocharger system 620 includes a high-pressure turbine 622 and a high-pressure compressor 624 coupled to each other via a high-pressure drive shaft 626. The low-pressure turbocharger system 640 includes a low-pressure turbine 642 and a low-pressure compressor 644 coupled to each other via a low-pressure drive shaft 646. The exhaust gas discharged from the exhaust manifold 608 is routed to the high-pressure turbine 622 through a first exhaust channel 612. The exhaust gas passing through the high-pressure turbine 622 is routed to the low-pressure turbine 642 via a second exhaust channel 618. In alternative embodiments, the second exhaust channel 618 may also receive exhaust gas routed via a bypass channel 614 placed between the inlet and outlet of the high-pressure turbine 622. In some embodiments, a valve 616 may be placed in the bypass channel 614 for regulating the amount of bypassed exhaust gas. The high-pressure turbine 622 is driven to rotate by exhaust gas supplied from the first exhaust channel 612. The low-pressure turbine 642 is driven to rotate by the exhaust gas supplied from the second exhaust channel 618. The exhaust gas passing through the low-pressure turbine 642 may be discharged directly to the environment via a third exhaust channel 658. In alternative embodiments, the exhaust gas may be re-circulated to the intake manifold 606 of the engine 602.

Further referring to FIG. 9, during operation, the low-pressure turbine 642 drives the low-pressure compressor 644 to rotate through the low-pressure drive shaft 646. The low-pressure compressor 644 compresses input air flow received from a first intake channel 632 and provides intermediate air flow with a boosted air pressure to a second intake channel 634. The intermediate air flow is further compressed by the high-pressure compressor 624 which is driven to rotate by the high-pressure turbine 622 through the high-pressure drive shaft 626. The high-pressure compressor 624 provides output air flow with further boosted air pressure to the intake manifold 606 via a third intake channel 654. In some embodiments, the intermediate air flow in the second intake channel 634 may be routed to the third intake channel 654 via a bypass channel 636. In some embodiments, a valve 638 may be placed in the bypass channel 636 for regulating the amount of bypassed air flow.

Further referring to FIG. 9, the low-pressure stage turbocharger system 620 may include a low-pressure thrust bearing 648. The low-pressure thrust bearing 648 is attached to the low-pressure drive shaft 646 for supporting axial thrust load 662 applied to the low-pressure thrust bearing 648. In one implementation, the axial thrust load 662 is a net thrust load which may include a compressor back-surface thrust load 664 component generated due to the leakage air flow at the back surface 643 of the low-pressure compressor 644. In one implementation, the low-pressure compressor 644 is configured with one or more flow restriction structures 649 at the back surface 643 of the low-pressure compressor 624. The one or more flow restriction structures 649 are similar to the flow restriction structures 224 as described above with reference to FIGS. 1-3. During rotational movement of the low-pressure compressor 644, a portion of the intermediate air flow is prevented from entering into the back surface 643 of the low-pressure compressor 644, such that the axial thrust load 664 applied to the low-pressure thrust bearing 648 is reduced. As a result, over-wearing problems of the low-pressure thrust bearing 648 can be avoided and/or the life of the low-pressure thrust bearing 648 can be extended or prolonged.

Further referring to FIG. 9, in one implementation, the high-pressure stage turbocharger system 620 may further include a high-pressure thrust bearing 628. The high-pressure thrust bearing 628 is attached to the high-pressure drive shaft 626 for supporting the axial thrust load 666 applied to the high-pressure drive shaft 626. In one implementation, the axial thrust load 666 is a net thrust load which may include a compressor back-surface thrust load 668 component. The net thrust load 666 and the compressor back-surface thrust load 668 point to the same direction which may be in parallel to a rotational axis of the high-pressure drive shaft 626. In the illustrated embodiment, the high-pressure compressor 624 may be optionally or additionally configured with one or more flow restriction structures 629. The one or more flow restriction structures 629 may be similar to the flow restriction structures 224 that have been described above with reference to FIGS. 1-3. During rotational movement of the high-pressure compressor 624, the one or more flow restriction structures 629 function to prevent at least a portion of the output air flow from entering the back surface 623 of the high-pressure compressor 624. The reduced amount of leakage air at the back surface 623 of the high-pressure compressor 624 causes a reduction of the axial thrust load 668 and the net thrust load 666 applied to the high-pressure thrust bearing 628. As a result, over-wearing problems of the high-pressure thrust bearing 628 can be avoided and the life of the high-pressure thrust bearing 628 can be extended or prolonged.

In alternative embodiments, the one or more flow restriction structures 649 provided at the back surface 643 of the low-pressure compressor 644 and/or the one or more flow restriction structures 629 provided at the back surface 623 of the high-pressure compressor 624 can be further modified for balancing purpose of the low-pressure compressor 644 and the high-pressure compressor 624 respectively. For example, the one or more flow restriction structures 629, 649 can be removed with material for balancing. Furthermore, the one or more flow restriction structure 629, 649 can also be added with material for balancing.

FIG. 10 illustrates a schematic block diagram of another two-stage turbocharger system 700 in accordance with an exemplary embodiment of the present disclosure. The two-stage turbocharger system 700 is similar to the two-stage turbocharger system 600 described above with reference to FIG. 9. Thus, similar elements will not be described in more detail in this embodiment. In the illustrated embodiment, the two-stage turbocharger system 700 further includes a bypass channel 656 which is in flow communication with the high-pressure compressor 624 and the low-pressure turbine 642. More specifically, in one embodiment, a first end 657 of the bypass channel 656 is coupled to the third intake channel 654 coupled between the high-pressure compressor 624 and the intake manifold 606 of the internal combustion engine 602. A second end 659 of the bypass channel 656 is coupled to the back surface 645 of the low-pressure turbine 642. The air flow at the back surface 645 of the low-pressure turbine 642 generates a turbine back-surface thrust load 665 which is a thrust load component of the net thrust load 662. In the illustrated embodiment, the turbine back-surface thrust load 665 is opposite to the compressor back-surface thrust load or the net thrust load 662.

Further referring to FIG. 10, in one implementation, during operation, the bypass channel 656 diverts a portion of the output air flow flowing in the third intake channel 654 to the back surface of the low-pressure turbine 642. More specifically, in the illustrated embodiment, the diverted air flow comes out from a combination of the air flow from the high-pressure compressor 624 and the bypass channel 636. In another embodiment, the diverted air flow may optionally directly come from the immediate output of the high-pressure compressor 624. The diverted air flow helps to increase the air pressure at the back surface 645 of the low-pressure turbine 642 which in turn increases the turbine back-surface thrust load 665 pointing from the low-pressure compressor 644 side to the low-pressure turbine 642 side. Because the net axial thrust load 662 points from the low-pressure turbine 642 side to the low-pressure compressor 644 side, thus, increasing the thrust load at the back surface 645 of the low-pressure turbine 642 can reduce the net axial thrust load 662 applied to the low-pressure thrust bearing 648. As a result, over-wearing problems of the low-pressure thrust bearing 648 can be avoided and/or the life of the low-pressure thrust bearing 648 can be extended or prolonged.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. Similarly, the various method steps and features described, as well as other known equivalents for each such methods and feature, can be mixed and matched by one of ordinary skill in this art to construct additional assemblies and techniques in accordance with principles of this disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A compressor comprising: a plurality of blades; a hub defining a front surface and a back surface, the plurality of blades being arranged in a predefined manner on the front surface for receiving input air flow at a first pressure and compressing the input air flow to provide an output air flow at a second pressure higher than the first pressure; and a first flow restriction member provided at the back surface of the hub, the first flow restriction member configured for preventing at least a portion of the output air flow at the second pressure from entering into the back surface of the hub to reduce an air pressure at the back surface of the hub.
 2. The compressor of claim 1, wherein the first flow restriction member is formed integral with the back surface of the hub.
 3. The compressor of claim 1, wherein the first flow restriction member is detachably coupled to the back surface of the hub.
 4. The compressor of claim 1, wherein the first flow restriction member extends from the back surface of the hub along a direction substantially parallel to a rotation axis that the hub rotates therewith, the first flow restriction member operates to deflect the air flow entering into the back surface of the hub and create a pressure difference at two sides of the first flow restriction member.
 5. The compressor of claim 1, wherein the first flow restriction member extends along a circumferential direction to form a ring-shaped member protruding backwardly from the back surface of the hub, the ring-shaped first flow restriction member divides the back surface into at least a first region and a second region, the first region is adjacent to the edge of the hub and the second region is adjacent to a rotational axis of the hub, wherein the first region has an air pressure higher than that of the second region during rotational movement of the hub.
 6. The compressor of claim 1, wherein the first flow restriction member comprises: a first surface extending substantially perpendicular to the back surface of the hub, the first surface deflecting at least a portion of the output air flow at the second pressure that enters into the back surface from a first direction to a second direction; and a second surface connecting to the first surface, the second surface extending substantially parallel to the back surface, the second surface further deflecting the air flow from the second direction back to the first direction.
 7. The compressor of claim 6, wherein the first restriction member further comprises a third surface connecting to the second surface and the back surface, the third surface extending substantially perpendicular to the back surface, the third surface further deflecting the air flow from the first direction to a third direction which is opposite to the second direction.
 8. The compressor of claim 6, wherein the compressor is capable of being enclosed in a housing which comprises a first wall running substantially parallel to the first surface of the first flow restriction member, the first wall and the first surface defines a flow channel which has a first dimension when the first flow restriction member is stationary with respect to the hub and a second dimension when the first restriction member is rotating with the hub, wherein the second dimension is smaller than the first dimension due to a centrifugal force applied at the first flow restriction member when the hub is rotating.
 9. The compressor of claim 1, wherein the back surface of the hub is further provided with a second flow restriction member constructed substantially similar to the first flow restriction member, the first and second restriction members are spaced apart along a radial direction of the compressor.
 10. A turbocharger system for an internal combustion engine, the turbocharger system comprising: a turbine in flow communication with an exhaust manifold of the internal combustion engine for receiving exhaust gas discharged from the exhaust manifold and being driven to rotate by the exhaust gas; a compressor coupled to the turbine through a drive shaft, the compressor driven to rotate by the drive shaft in response to a rotation of the turbine for supplying pressurized air to an intake of the internal combustion engine; and a thrust bearing attached to the drive shaft for supporting at least a thrust load applied along an axial direction of the drive shaft; wherein the compressor comprises a hub defining a back surface provided with a flow restriction member, the flow restriction member deflects a flow path of at least a portion of the pressurized air entering into the back surface at least once to create a pressure difference between two areas at least partially defined by the flow restriction member, and the pressure difference created by the flow restriction member causes the thrust load applied along the axial direction of the drive shaft to be reduced.
 11. The turbocharger system of claim 10, wherein the compressor is at least partially enclosed within a compressor housing, the flow restriction member is capable of being moved along a radial direction of the hub in response to a rotational movement of the hub and the dimension of a flow channel defined between the flow restriction member and a wall of the compressor housing is reduced due to the radial movement of the flow restriction member to reduce the amount of the pressurized air entering to the back surface of the hub.
 12. The compressor of claim 10, wherein the flow restriction member extends from the back surface along a direction substantially parallel to a rotation axis that the hub rotates therewith, the flow restriction member operates to deflect the pressurized air entering into the back surface of the hub and create a pressure difference at two sides of the flow restriction member.
 13. The compressor of claim 10, wherein the flow restriction member extends along a circumferential direction to form a ring-shaped member protruding backwardly from the back surface of the hub, the ring-shaped flow restriction member divides the back surface into at least a first region and a second region, the first region is adjacent to the edge of the hub and the second region is adjacent to the drive shaft, wherein the first region has an air pressure higher than that of the second region during rotational movement of the hub.
 14. A multi-stage turbocharger system for an internal combustion engine, the multi-stage turbocharger comprising: a low-pressure stage comprising: a low-pressure turbine; and a low-pressure compressor capable of being driven by the low-pressure turbine to compress input air flow at a first air pressure and provide intermediate air flow at a second air pressure higher than the first air pressure; and a high-pressure stage comprising: a high-pressure turbine; and a high-pressure compressor placed downstream of the low-pressure compressor, the high-pressure compressor capable of being driven by the high-pressure turbine to compress at least a portion of the intermediate air flow provided from the low-pressure compressor and supply output air flow at a third air pressure higher than the second air pressure to an intake of the internal combustion engine; wherein the high-pressure compressor is in flow communication with the low-pressure turbine.
 15. The multi-stage turbocharger system of claim 14, wherein the low-pressure stage further comprises: a low-pressure drive shaft for coupling the low-pressure turbine to the low-pressure compressor; and a low-pressure thrust bearing attached to the low-pressure drive shaft for supporting at least a thrust load applied along an axial direction of the low-pressure drive shaft; wherein at least a portion of the output air flow provided from the high-pressure compressor is diverted to a back surface of the low-pressure turbine to increase the air pressure at the back surface of the low-pressure turbine and reduce the thrust load applied along the axial direction of the low-pressure drive shaft.
 16. The multi-stage turbocharger system of claim 14, wherein the low-pressure compressor comprises a flow restriction member provided at a back surface of the low-pressure compressor, the flow restriction member deflects a flow path of at least a portion of the intermediate air flow entering into the back surface of the low-pressure compressor at least once to create a pressure difference between two areas at least partially defined by the flow restriction member, and the pressure difference created by the flow restriction member causes the thrust load applied along the axial direction of the low-pressure drive shaft to be reduced.
 17. The multi-stage turbocharger system of claim 16, wherein the flow restriction member extends along a circumferential direction to form a ring-shaped member protruding backwardly from the back surface of the low-pressure compressor, the ring-shaped flow restriction member divides the back surface of the low-pressure compressor into at least a first region and a second region, the first region is adjacent to the edge of the low-pressure compressor and the second region is adjacent to the low-pressure drive shaft, wherein the first region has an air pressure higher than that of the second region during rotational movement of the low-pressure compressor.
 18. The multi-stage turbocharger system of claim 16, wherein the flow restriction member is capable of being moved along a radial direction of the low-pressure compressor in response to a rotational movement of the low-pressure compressor and the dimension of a flow channel defined between the flow restriction member and a wall of a compressor housing is reduced due to the radial movement of the flow restriction member to reduce the amount of the intermediate air flow entering to the back surface of the low-pressure compressor.
 19. The multi-stage turbocharger system of claim 14, wherein the low-pressure compressor comprises a first flow restriction member and a second flow restriction member provided at a back surface of the low-pressure compressor, the first and second restriction members are spaced apart along a radial direction of the back surface of the low-pressure compressor, and the first and second restriction members operate to deflect a flow path of at least a portion of the intermediate air flow entering into the back surface of the low-pressure compressor at least once to create a pressure difference between areas at least partially defined by the first and second flow restriction members, and the pressure difference created by the flow restriction member causes the thrust load applied along the axial direction of the low-pressure drive shaft to be reduced.
 20. The multi-stage turbocharger system of claim 18, wherein the first and second flow restriction members extend along a circumferential direction to form ring-shaped members protruding backwardly from the back surface of the low-pressure compressor, the ring-shaped flow restriction members divide the back surface of the low-pressure compressor into at least a first region, a second region, and a third region, wherein the air pressure of the second region is smaller than that of the first region and greater than that of the third region. 